European Patent Application No. 0 477 400 A1 discusses a system for an adaptive mechanical tolerance compensation, which is effective in the lift direction and intended for a path transformer of a piezoelectric actuator for a fuel injector. In this case, the actuator acts on a master (transmitting) piston, which is connected to an hydraulic chamber, and a slave (receiving) piston, which moves a mass to be driven and positioned, is moved via the pressure increase in the hydraulic chamber. This mass to be driven is a valve needle of a fuel injector, for example. The hydraulic chamber is filled with an hydraulic fluid. When the actuator is deflected and the hydraulic fluid in the hydraulic chamber is compressed, a small portion of the hydraulic fluid leaks at a defined leakage rate. This hydraulic fluid is replenished in the rest phase of the actuator.
German Patent Application No. 195 00 706 A1 discusses an hydraulic path transformer for a piezoelectric actuator of a fuel injector, which is positioned between the actuator and a valve needle of the fuel injector. A master piston and a slave piston are arranged on a common axis of symmetry, and an hydraulic chamber lies between the two pistons. A spring, which presses the master cylinder and the slave piston apart, is located in the hydraulic chamber, the master piston being prestressed in the direction of the actuator and the slave piston in a working direction against a valve needle. When the actuator transmits a lifting movement to the master cylinder, this lifting movement is transmitted to the slave piston by the pressure of an hydraulic fluid in the hydraulic chamber, since the hydraulic fluid in the hydraulic chamber is not compressible and only a small portion of the hydraulic fluid is able to escape during the short duration of a lift through ring gaps between the master piston and a guide bore, and a slave piston and a guide bore.
In the rest phase, when the actuator does not exert any compressive force on the master cylinder, the master piston and the slave piston are pushed apart by the spring, and, due to the produced vacuum pressure, the hydraulic fluid enters the hydraulic chamber via the ring gaps and refills it. In this way, the path transformer automatically adapts to linear deformations and pressure-related expansions of a fuel injector.
Disadvantageous in this related art is that the path transformer gets very hot from the waste heat of an internal combustion engine. The path transformer is located in a region of the fuel injector that lies deep in an installation bore once the fuel injector is installed and thus in close proximity to the combustion chamber. In the rest phases of the actuator, the fuel may evaporate and thus cause a failure of the fuel injector, since the evaporated fuel is compressible and the valve needle is not opened for that reason.
This danger exists in particular after a hot internal combustion engine has been shut off. The fuel-injection system then loses its pressure, and the fuel evaporates particularly easily. This may have the result that in a renewed effort to start the internal combustion engine the lifting movement of the actuator is no longer transmitted to a valve needle and the fuel injector no longer functions.